1. Field of the Invention
This invention relates to a pitch based carbon fiber and, more particularly, to a high compressive strength pitch based carbon fiber having a uniform crystal structure.
2. Prior Art
Heretofore, PAN and Rayon based carbon fibers made of polyacrylonitrile and rayon, respectively, are finding extensive applications as carbon fibers having high tensile strength and high tensile modulus elasticity, for instance. However, the precursors of these fibers are expensive and are inferior in the carbonization yield. Therefore, it suffers economical problems. Also, it is very difficult to produce high elastic modulus products with a tensile elastic modulus of 60 ton/mm.sup.2 or above from the PAN or Rayon based carbon fibers noted above.
Recently, extensive researches and developments concerning carbon fibers obtainable from petroleum or coal tar pitches are being conducted from the standpoint of the facts that the raw materials are inexpensive and permit higher carbonization yield to be obtained. It is known that in order to obtain high strength, high elastic modulus carbon fibers, for instance, from petroleum or coal tar pitches or other carbonaceous materials, it is necessary to obtain a liquid crystal pitch with an optically anisotropic phase content of 95% or above, substantially 100%.
As examples, Japanese Patent Laid-Open No. 54-55625 shows a method of obtaining a liquid crystal pitch with an optically anisotropic phase content of substantially 100% through the long thermal pyrolysis, polymerization and/or condensation reactions by combining bubbling with inert gas and stirring, and Japanese Patent Laid-Open No. 54-160427 shows a method of obtaining a liquid crystal pitch with an optically anisotropic phase content of substantially 100% in a solvent extraction process.
Also, Japanese Patent Publication No. 61-38755 shows a method of obtaining a liquid crystal pitch with substantially 100% optically anisotropic phase content by heat treating the precursor pitch to produce an optically anisotropic phase-containing pitch and then subjecting the optically anisotropic phase-containing pitch to specific gravity difference separation. The obtainable liquid crystal pitch with substantially 100% optically anisotropic phase content has a softening point of 230.degree. to 320.degree. C., which is very low compared to those liqiud crystal pitches for pitch based carbon fibers disclosed in the above-mentioned Jpanese Patent Laid-Open Nos. 54-55625 and 54-160427, and thus it can be stably spun at a temperature of from 280.degree. to 380.degree. C.
The liquid crystal pitch obtained in this way is melt spun at a temperature of 280.degree. to 380.degree. C. to obtain pitch fibers, which are then oxidatively stabilized at 200.degree. to 350.degree. C. and then carbonaized and graphitized at a high temperature of around 500.degree. to 3,000.degree. C. in an inert gas atmosphere to obtain carbon fibers. If necessary, an elongation treatment is carried out simultaneously with the stabilization and carbonization processes.
The pitch based carbon fibers which are obtained in the above way, have been improved in the orientation of the constituent molecules of the liquid crystal pitch. Thus, a high degree of the axial orientation of the molecules isattainable. Further, it is possible to obtain promotion of the graphitization and increase of the crystal size owing to the polymerization and condensation reactions of methyl and naphthenic groups in the liquid crystal pitch caused by the high temperature carbonaization. Such pitch based carbon fibers show not only higher tensile strength and modulus but higher thermal and electric conductivities.
While the pitch based carbon fibers provide higher tensile strength and especially tensile modulus caused by their higher graphitizabity, they have a significant problem that their compressive strength is extremely reduced with improvement of the crystallization degree. With current pitch based carbon fibers, the compressive strength is as low as 66 to 83 kg/mm.sup.2 with a tensile elastic modulus of 50 ton/mm.sup.2 and 58 to 66 kg/mm.sup.2 with a tensile elastic modulus of 70 ton/mm.sup.2.
With recent progress of technology, high performance carbon fibers, which are not only light in weight and highly strong at tension but also highly strong at compression, and which are nevertheless low in cost, are demanded for composite materials in the fields of space, aircrafts, vehicles, construction and various other industrial fields.
The inventors conducted extensive researches and experiments with an aim of obtaining carbon fibers, which can meet such demands, particularly those having high tensile elastic modulus and high compressive strength. As a result, it was found that crystal fibriles constituting carbon fibers have non-uniform crystal structure distribution over a crystal thickness range of 15 to 300%, sometimes 10 to 400%, of the average thickness Lav., thus leading to non-uniform stress concentration at compression. The compressive strength, therefore, was low.
Thus, if the crystal fibriles constituting carbon fibers have thicknesses held within a narrow range with respect to the average thickness Lav. and thus have high uniformity of crystal structure, it is possible to obtain uniform compressive strength distribution over each crystal fibril to obtain high compressive strength.
The inventors found that the uniformity of carbon fiber fibriles depends on the uniformity of the molecular weight distribution of the precursor liquid crystal pitch and also on the uniformity of the orientation of pitch constituent molecules promoted by the application of multiple stage high shear spinning, and also that the compressive strength of pitch based carbon fibers can be improved not only through the control of the temperature of heat treatment of the pitch fibers but also the molecular weight distribution of the liquid crystal pitch and the process of spinning of the pitch fibers.